gdb/tui: avoid extra refresh_window on vertical scroll

gdb/tui: avoid extra refresh_window on vertical scroll

While working on the previous couple of patches I noticed that when I
scroll the src and asm windows vertically, I get two refresh_window
calls.

The two calls can be traced back to
tui_source_window_base::update_source_window_as_is, in here we call
show_source_content, which calls refresh_window, and then
update_exec_info, which also calls refresh_window.

In this commit I propose making the refresh_window call in
update_exec_info optional.  In update_source_window_as_is I'll then
call update_exec_info before calling show_source_content, and pass a
flag to update_exec_info to defer the refresh.

There are places where update_exec_info is used without any subsequent
refresh_window call (e.g. when a breakpoint is updated), so
update_exec_info does not to call refresh_window in some cases, which
is why I'm using a flag to control the refresh.

With this changes I'm now only seeing a single refresh_window call for
each vertical scroll.

There should be no user visible changes after this commit.